The present invention relates to a negative pressure control apparatus for automatically changing a a negative pressure in accordance with a load of an engine and, more particularly, to a negative pressure control apparatus for controlling an exhaust gas recirculation control valve (to be referred to as an EGR control valve for brevity hereinafter) for a diesel engine.
Recently, measures to restrict exhaust gases have become necessary for diesel engines as well as for gasoline engines. An EGR control system, for example, is used for a diesel engine.
Unlike a gasoline engine, a diesel engine does not have a pressure source which can change pressure in accordance with the load of the engine. For this reason, an EGR control system for a diesel engine must have a separate pressure source so as to control the operation of an EGR control valve for the purpose of adjusting the flow rate of the exhaust gas. In view of this requirement, an EGR control system has been proposed which has as a pressure source a combination of a vacuum pump and a negative pressure control apparatus. Such a negative pressure control apparatus has a negative pressure chamber connected to the EGR control valve. The negative pressure in the negative pressure chamber must be changed in accordance with the load of the diesel engine. For this purpose, the negative pressure chamber of the apparatus is connected to a vacuum chamber in accordance with the load of the engine, so that the interior of the negative pressure chamber may be kept at a suitable negative pressure. In the negative pressure control apparatus of the type described above, the negative pressure in the negative pressure chamber must be controlled in a precise manner. The control apparatus must be assembled with high precision and must also be easy to assemble. The apparatus should preferably allow adjustment after assembly.